1. Field of Invention
This invention relates to safety devices, specifically to a mechanism for prevention of flareup in barometric-type, wick-fed liquid fuel burners.
2. Discussion of Prior Art
In wick-fed liquid fuel burners, such as kerosene heaters, liquid fuel from a fuel chamber is supplied to a wick which is exposed to the oxygen of the atmosphere. Once the wick has been ignited, flame intensity and heat generation are controlled by positioning the wick with respect to a wick-receiving combustion chamber.
A common type of kerosene heater is the barometric style, in which gravity causes liquid fuel to be delivered to a horizontal fuel chamber from a vertically-oriented removable tank inserted into the fuel chamber. The flow of fuel from the removable tank into the fuel chamber is governed by a barometric valve in the cap on the removable tank, which, in normal operation, maintains the level of the fuel in the fuel chamber at the level of the barometric valve. A partial vacuum above the fuel in the removable tank prevents the fuel from flowing into the fuel chamber until the fuel level in the fuel chamber drops below the barometric valve, which allows air to enter the removable tank. As air enters the removable tank through the barometric valve, fuel in the removable tank flows into the fuel chamber until its level in the fuel chamber rises and covers the barometric valve in the removable tank cap, at which point fuel flow from the removable tank will cease. The barometric valve consists of a spring-loaded plunger, which has an enlarged head at one end. When the removable tank is inserted into the fuel chamber, the plunger head strikes a pin located in the fuel chamber, which pushes the plunger back, allowing the fuel in the removable tank to be in fluid communication with the fuel chamber. When the tank is removed, the action of the spring on the plunger head forces it against the opening in the tank cap, sealing the opening and preventing fuel from escaping from the tank.
Various improvements have been made to such burners which make them safer to operate. For example, tip-over shut-off mechanisms, manual shut-off devices, and low-level O.sub.2 detectors have been employed. However, these burners continue to cause fires that result in death, injury, and property loss. These fires are caused, because, under certain conditions, fuel can overflow the fuel chamber. When the flooded fuel ignites, the result is an uncontrolled fire, or flareup.
The most common reason for fuel overflow is the accidental use of fuels with high vapor pressures. Examples of such fuels are gasoline, naphtha, and inferior kerosene, which has a low flash point. In a barometric heater, overflow of fuel from the fuel chamber can occur if the partial vacuum in the removable tank is lost. As the temperature of the heater and its surroundings increases, the vapor pressure of the fuel in the removable tank increases and, under certain conditions, allows fuel to escape from the removable tank at a rate greater than the rate of burning of the fuel. The excess fuel eventually overflows the fuel chamber an spills onto the top of the fuel chamber and the surface supporting the heater. The spilled fuel can then ignite, causing an uncontrolled fire. A second way that the partial vacuum in the barometric heater's removable tank can be lost is by air entering through compromise of the integrity of the removable tank.
There are safety devices that drop the wick down, thereby extinguishing the flame, if the burner tips over or experiences excessive vibration, or if abnormal combustion is detected. Other safety devices detect high levels of CO.sub.2 and low levels of O.sub.2 in the vicinity of the heater, and use these to control burning rates. Still others regulate the position of the wick during the ignition and extinguishing operations of the heater to prevent excessive flaming during these operations. Examples are shown in U.S. Pat. Nos. 4,363,620, issued Dec. 14, 1982 to Nakamura, 4,812,831, issued Oct. 10, 1989 to Fujimoto, 4,797,088, issued Jan. 10, 1989 to Nakamura, and 5,165,883, issued Nov. 24, 1992 to Van Bemmel. In some cases, the safety devices require the use of electrical power and electronic circuitry for actuation.
It has been suggested in two publications ("Kerosene Heater Fires: Barometric Type," R. Henderson et al., Fire Marshals Bulletin (National Fire Protection Association), Vol. 87-5, p. 8 (1987); "Barometric Kerosene Heaters," R. Henderson, Fire and Arson Investigator (International Association of Arson Investigators), Vol. 39, No. 3, p. 26 (1989)) to make the size of the removable tank of barometric kerosene heaters comparable in volume to that of the fuel chamber so that flooding of the fuel chamber will not occur. To implement this suggestion, either the capacity of the removable tank must be reduced, or alternatively, that of the fuel chamber must be increased. However, reducing the capacity of the removable tank will reduce the burn time accordingly, and possibly affect the marketability of the heaters. Increasing the capacity of the fuel chamber will require that new tanks be designed and implemented.
Also, it has been suggested that a float device be introduced into the fuel chamber to be used to activate the automatic wick extinguishing mechanism, and a sight gauge be present to show dangerous fuel levels in the fuel chamber. Introduction of such a float device would also require that the fuel chamber be redesigned, as discussed above. Although some burners have sight gauges in the fuel chamber, the sight gauges are used only to indicate whether or not fuel is present, and not when dangerous fuel levels are present in the fuel chamber.
In addition it was proposed that a tank block-out device be installed, in which a float in the reservoir would push on a pin that could move should the removable tank be withdrawn from the heater. Once again, such a device would require a redesigning of the fuel chamber and insertion of moving parts inside a somewhat restricted space. Also, this type of device would not prevent the entire contents of the removable tank from flowing into the fuel chamber, since it becomes operable only after the removable tank has been withdrawn.
U.S. Pat. No. 5,080,578, issued Jan. 14, 1992 to Josephs, claims that its device controls flareup in wick-fed liquid fuel burners by a) cutting off the flow of fuel to the wick in response to excessive heat by blocking a fuel line, and b) withdrawing the wick into the wick chamber when sensing excessive heat. However, Joseph's device has several disadvantages:
a) Excessive heat must be generated near the sensors before the flow of fuel is interrupted, or the wick is withdrawn. Therefore, since flareup is not prevented, the device only limits the spread of excessive flames after flareup has already occurred.
b) Excessive heat sensing devices must be near the area where uncontrolled burning is taking place due to overflow of fuel. Often the path that the overflowing fuel takes is random and flareup may not initially occur near the heat sensors.
c) The device is not applicable to barometric liquid fuel burners--the most common wick-fed liquid fuel burners in use --because these burners do not have liquid fuel burners.
d) From the onset of flareup in wick-fed liquid fuel burners, fire is present outside the wick, and therefore retracting the wick does not affect the flareup process.
The device of the above-referenced related patent of Henderson and Lightsey prevents flareup by activating a wick-extinguishing mechanism when the presence of excess fuel is detected in the fuel chamber. While this device has much merit, it should be noted that to be effective it requires activation of a second entity, the automatic wick extinguishing mechanism. Should that mechanism fail to respond, due to tar buildup on the wick or a mechanical problem, flareup can still occur.
The device of the above-referenced application of Henderson prevents flareup by lifting the removable tank, thereby shutting off the barometric valve and stopping fuel flow from that tank when excess fuel is present in the fuel chamber. For this device to work, it is necessary for a spring to lift the removable tank and its contents of liquid fuel, the total weight of which can be up to some ten pounds. Accordingly, should the spring lose strength, or should the removable tank become hindered in its upward movement, this device may not be able to prevent flareup.